Log into the Last.fm web site, scroll to the bottom of the home page and under "Account" click "Settings". This will bring up all your account details. Along the top there is a menu bar, one item is "Applications", click that.
Under this there is a new item "Spotify Scrobbling", click that. You can then enter your Spotify user name and password. Anything you now play on Spotify using that account will be Scrobbled, including your phone, ipad, Spotify Connect, desktop or web app.
For some reason the Android Spotify App still has an option to enable scrobbling, but nowhere to put in your account details. Just turn this off - it is no longer needed.